The present invention relates to electrical devices and more particularly to signal generators which are touch sensitive and produce output electrical signals in response to an input stimulus of force.
Products having a wide range of applicability are in use today and many of them either have a keyboard or could be utilized more conveniently if a suitable keyboard were available. Machines such as calculators, typewriters, computers, telephones and home appliances to mention only a few have the common requirement for a precise input which determines the actual functioning of the particular device.
In the case of any typewriters and calculators, for example, mechanical keys are used to input the machine. Such systems are tried and tested and have an established acceptance in some markets. However, the individual key input type device tends to involve a relatively large number of moving mechanical parts which is often relatively bulky and heavy. In addition, such systems require servicing and cleaning at periodic intervals in order to maintain acceptance operating efficiency. Obviously, as the number and complexity of the linkages increases, such as is found in some typewriters, the cost of manufacture and assembly becomes significant.
The art in this field is developed somewhat and for certain applications, particularly in home appliances, automobiles and calculators, the use of individual keys or buttons continues to be used to provide a mechanical input which is quickly converted into electrical signals which are then determinative of the operation of the machines. Such use of buttons has been a considerable improvement in certain applications although they still contain inherent disadvantages which seem to center about the input button portion of the device. For example, the buttons have a tendency to jam or break and also since they are undergoing physical displacement, they provide locations through which dirt and foreign particles can enter into the control system. The latter problem has been minimized considerably by the use of a protective layer of flexible material which is typically rubber or other suitable plastic. Such covers can be very effective in minimizing the amount of foreign material which accumulates in and around the buttons. The covers do not, however, solve the problem of wear and maintenance associated with the contact points inherent in the button system.
Another variation to the button control panel is the dial device such as is found on many telephones. With the use of a dial, discrete inputs can be provided while avoiding many of the disadvantages described above associated with either the buttons or the keys. However, the dial is a relatively large and bulky mechanical system that does rely on contacts and is subject to both wear and breakage.
Alternatively some applications involving microwave ovens and elevators have found use for capacitive or resistive type switches which experience a change in electrical characteristics when activated. Such devices were well received although they are considered erratic during unusual circumstances such as overvoltages or excessive heating.